🤖 Fast mode unavailable toast: wrong reason for models without fast mode, silent on the no-provider screen

[ThomasK33]

Problem: The fast-mode toast added in #5744 (#5693) has two gaps:

1. For models that never support fast mode (for example Gemini), the toast blames the provider route ("for example through a gateway or a custom base URL"). The real reason is that the model has no fast mode.
2. On the new-workspace screen with no provider configured, no composer is mounted, so the toast event (ANALYTICS_REBUILD_TOAST) has no listener and the shortcut stays silent. That screen already shows "Configure providers".

Fix idea: distinguish "this model has no fast mode" from "this route does not support it" in getFastModeProvider's callers. Consider a toast host that stays mounted without a composer, and maybe rename the event, which is a de-facto shared feedback event.

Priority P3. Size S-M. Follow-up of #5744.

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[ThomasK33]

Item 1 (wrong reason for models without fast mode) is fixed in #5756.

Item 2 (no toast on the new-workspace screen with no provider) stays out of scope, on purpose. That screen mounts no composer, so ANALYTICS_REBUILD_TOAST has no listener there. A fix needs a toast host that stays mounted without a composer. That is a new app-wide surface, and the screen already shows "Configure providers", which tells the user what to do. This issue stays open for item 2.

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[ThomasK33]

The fix for item 1 (the toast names the real reason) is on #5756, which is blocked and not merged. It has not shipped, so this issue stays open. Item 2 (no toast on the no-provider screen) stays out of scope, as explained above.

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[ThomasK33]

Status after #5756 (merged as 752f962a69):

1. Item 1 shipped in 🤖 fix: Ctrl+I stays out of the command palette, fast mode toast names the real reason #5756. The fast mode shortcut now says "this model has no fast mode" for models with no fast mode on any route, such as Haiku 4.5 and Gemini. It keeps the route wording for models that have fast mode elsewhere. Known exceptions, where the code cannot tell the model's identity (Copilot-hosted Claude, openai-compatible custom providers, unknown Coder instances), are tracked in 🤖 Fast mode toast: unknown model identity should give the route reason #5764.
2. Item 2 remains open: the shortcut shows no toast on the no-provider screen. That screen mounts no composer, so no toast listener exists there. My earlier comment on this issue explains why 🤖 fix: Ctrl+I stays out of the command palette, fast mode toast names the real reason #5756 left it out.

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